I. Pavlicsek

Nuclear excitation by γ rays of radioactive sources

Nuclear isomers of stable nuclei were excited by inelastic resonance scattering of Compton-scattered γ rays from high-activity46Sc,60Co,42Pr and24Na radioactive sources. By counting isomeric transition rates, activation cross sections for thirty-five isomers were determined, some of them not previously published. Values of these cross sections lie between 0.003 and 214 μbarn. The partial level widthsμ =gΓ0ΓiΓ−1 for the transitions leading from the activation to the metastable levels, estimated for twelve activation levels belonging to eight nuclei, are between 4·10−6 and 2·10−4 eV.

Photoexcitation of 189Osm and 193Irm. I. Excitation of 189Osm by low-energy x rays.

Production of 5.84 h {sup 189}Os{sup {ital m}} by 200 through 300 kV bremsstrahlung has been measured and attributed to population of four nuclear states between 69.54 and 275.91 keV. From the measurements and known level parameters, contributions to isomer production from individual levels have been separated, and the half-life of the 216.66 keV level has been determined as 77{plus_minus}10 ps. Besides nuclear resonance absorption (NRA), the first level has long been assumed to be also excited by another process called nuclear excitation by electron transition (NEET), via virtual photon exchange due to recombination of vacancies produced in the {ital K} shell by incident x rays. However, contribution from competing NRA cannot be separated, and the claimed dominance or even the existence of the NEET process has remained unconfirmed.

Investigation of fast neutron emission of spent fuel assemblies with CR-39 track etch detector

A method based on fast neutron detection with CR-39 track etch detector has been developed for monitoring burnup of spent reactor fuel assemblies.

Determination of the production rate of low intensity isomeric transitions

Abstract Flat 2 π and cylindrical 4 π multiwire proportional counters were built for counting low energy internal conversion electrons from the nuclear isomers 83m Kr, 103m Rh and 189m Os, induced by irradiation with high intensity 60 Co and 4 MeV bremsstrahlung sources. The β-decay of 176m Lu was recorded by a plastic scinillator. In this way higher sensitivities were attained than by detecting γ-rays or characteristic X-rays associated with the isomeric transitions, and the excitation of 189m Os by low energy 137 Cs and 300 kV X-ray sources also became detectable. Comparatively large isomeric activities produced by linac irradiation were standardized by a Ge spectrometer for calibrating proportional and scintillation counting.

Anticoincidence counting of low intensity isomeric transitions

A new detector system was developed to count the highly converted low-energy isomeric transitions of long-lived 189mOs and 193mIr produced by (γ, γ′) reactions upon irradiating stable elemental Os and Ir samples with gamma photons from a 200 kV X-ray generator and high-activity 137Cs and 60Co sources. This system consists of multiwire flat 2π proportional chambers for internal counting, with sensitive areas 90, 20 and 6 cm2, surrounded by two large 2π counters connected in anticoincidence to reduce background. Specific activities in the order of Bq g−1, corresponding to counting rates lower than 0.1 cpm, were obtained. Photoproduction of 193mIr was detected for the first time below an excitation energy of 2.75 MeV. Excitation of 189mOs by 200 keV endpoint energy X-rays due to an interaction via nuclear excitation by electronic transition (NEET) was also observed.

Integral cross section of the99Tc(γ,γ′)99mTc reaction at 4 MeV

Abstract99mTc production was studied with the aid of photoexcitation by a 4 MeV endpoint energy bremsstrahlung from the LINAC of the Institute of Isotopes, Budapest. The intensity of the γ-flux was monitored by disc-shaped natural indium plates, placed in front of and behind the small cylindrical aluminium holders containing TcO2 samples in a powder form. Isomeric activities were measured through the 140 keV γ-line by a Ge spectrometer. The integral cross section at 4 MeV was found to be 63.3±7.1 μbMeV, which can be considered reasonable compared to the corresponding value of 55.3 μbMeV established for the115In (γ,γ′)115mIn reaction. We also attempted the photoexcitation of99mTc by irradiation with γ-rays from a 1.5×1015 Bq60Co source, but no isomeric activity could be observed. This places the first activation level between 1.33 and 4 MeV.

Gamma-Ray Activation and Passive Neutron Methods for Nondestructive Assay of Spent-Fuel Assemblies

This paper presents a nondestructive method for determining burnup and fissile content. Indium-113 is activated to metastable state by hard gamma rays of a spent-fuel assembly, where the isomer activity produced is proportional to the fissile content. The application of a beryllium converter increases sensitivity by nearly two orders of magnitude and also reduces the time needed. Alternative or simultaneous detection of neutrons emitted by the spent fuel is also available: The produced {sup 116m} In activity is a function of burnup. Principles are verified by many experiments of WWR-SM assemblies. Axial and azimuthal gamma-ray and neutron profiles are also recorded to demonstrate the ability of the methods to detect burnup inhomogeneities. The advantages of the methods and the choice of target material are discussed. Applications for safeguards purposes are suggested.

Measurement of103mRh produced by the103Rh/γ, γ′ /103mRh reaction with liquid scintillation counting

A liquid scintillation counting technique was applied to measure the isotope103mRh /half life = 56.12 min/ which is difficult to detect because its γ-ray is of low energy and low emission probability. Tris-/2,4-pentanedionato/ rhodium /III/ /Rh/ acac/3/ was irradiated with bremsstrahlung of accelerated 3.2 MeV electrons by LINAC. The method has given a reliable calibration curve for the determination of103mRh radioactivity below Rh/acac/3 concentrations of 2 mM. The integrated cross section of103Rh/γ,γ′/103mRh determined by this method was found to be 6.8±3.4 μb MeV at 3.2 MeV.

Excitation of 83mKr by 60Co irradiation

The 1.83 h half-life isomeric activity of 83mKr induced by 7.4 PBq (200 kCi) 60Co irradiation has been measured by a 4π proportional counter within a large background of 85Kr decay. The main activation level has been identified at 1170 keV. The cross section for the 83Kr(γ,γ′)83mKr reaction integrated over this level has been determined relative to a known value for 115mIn production and found to be (3.2±0.7)10−26 cm2 eV, in good agreement with the value calculated from the level scheme.

Photoexcitation of 3.7 h176Lum and its astrophysical consequences

Gamma-induced excitation of the 3.7 h half-life176Lum was studied by high activity50Co and24Na γ-sources as well as bremsstrahlung from a 4 MeV electron linear accelerator. Photoactivation exhibited a steep rise in the energy region concerned, from 1.17 to 4 MeV.Observed abundance of the 3.6 × 1010y half-life176Lu was proposed to be used for determining the age of the neutron capture s-process in stellar nucleosynthesis. However, the situation is complicated by the β-radioactivity of the isomer. Photoactivation may reduce drastically the half-life in stellar interior, so that it would reach the half-life of the isomer at 3.5×108 K temperature, guaranteeing thermal equilibrium between ground and isomeric states. This makes176Lu unreliable as a cosmic chronometer.